1. Field of the Invention
The present invention relates to a sheet transport device and an image forming apparatus.
2. Description of the Related Art
Typically, an image forming apparatus is provided with a sheet transport device for transporting sheets. The sheet transport device having a sensor for detecting whether each sheet has passed a particular point in a paper path. This sensor is a photointerrupter (hereinafter referred to as a PI sensor) including a light-emitting element and a light-sensitive element. The light-emitting element has a light-emitting surface from which light is emitted while the light-sensitive element has a light-receiving surface for sensing the light emitted from the light-emitting surface. The sensor further includes an interposed element which can be inserted into and withdrawn from a space formed between the light-emitting element and the light-sensitive element.
When a sheet passes the point in the paper path where the PI sensor is mounted, the sheet goes into contact with the interposed element, forcing the interposed element from within the space between the light-emitting element and the light-sensitive element to the outside of the space. More specifically, when forced by the sheet, the interposed element swings by a specific angle and moves from within the space between the light-emitting element and the light-sensitive element to the outside thereof. When moved to the outside of this space, the interposed element does not interrupt the light emitted from the light-emitting surface, whereby the PI sensor detects that the sheet has passed the particular point in the paper path.
If no sheet passes the point in the paper path where the PI sensor is mounted, the interposed element does not swing and thus remains in the space formed between the light-emitting element and the light-sensitive element. In this case, the interposed element interrupts the light emitted from the light-emitting surface, whereby the PI sensor detects that no sheet has passed the particular point in the paper path.
Since the sheet is transported along the paper path in sliding contact with various elements provided therearound, such as a transport roller or a transport guide plate, a static charge is accumulated in the sheet as a result of friction. If a discharge of the accumulated static charge occurs directly at the PI sensor, then a breakdown of a control board of the PI sensor, for example, may potentially occur.
One approach to the solution of this problem is shown in Japanese Unexamined Patent Publication No. 2005-162342, which proposes the provision of a grounding plate attached to a PI sensor for protecting the sensor from a discharge of static electricity. Specifically, the grounding plate described in this conventional approach is a flat-shaped plate fitted to a paper path close to a light-emitting element and a light-sensitive element of the PI sensor, the grounding plate being arranged to cover the entirety of surfaces of the light-emitting element and the light-sensitive element facing the paper path. With this arrangement, the static electricity is discharged to the grounding plate so that the PI sensor is protected from the discharge of the static electricity.
However, the aforementioned grounding plate is flat-shaped so that the grounding plate must have a larger surface area than a total area of the surfaces of the light-emitting element and the light-sensitive element facing the paper path in order to protect the PI sensor from the discharge of the static electricity. Considering, in particular, a mechanical error which occurs when the grounding plate is mounted to the PI sensor, it is necessary that the grounding plate have a large surface area.
In addition, since the grounding plate is flat-shaped, a discharge of static electricity which occurs from a sheet may not directly reach the grounding plate but the PI sensor, if the surface area of the grounding plate generally equals the total area of the surfaces of the light-emitting element and the light-sensitive element facing the paper path. This is another reason why the grounding plate must have a large surface area to ensure that the discharge of static electricity would easily reach the grounding plate.
If, however, the surface area of the grounding plate is so increased, the space between the light-emitting element and the light-sensitive element will be reduced. This would make it difficult to smoothly insert and withdraw the interposed element into and out of the space between the light-emitting element and the light-sensitive element and, as a consequence, it will be difficult that the PI sensor properly detects whether the sheet has passed the particular point in a paper path.